1. Field of the Invention
This invention relates to microwave `balun` transformers, so called because of the transition they provide between balanced and unbalanced lines or systems.
2. Description of Related Art
A particular application of such transformers concerns cavity-backed antennas, in which, for example, a double spiral conductor mounted on a dielectric plate is backed by a cavity to take up power radiated backwards from the spiral. The cavity may be of such dimensions that a reflecting wall opposite to the spiral reflects the backward signal with such phase as to reinforce the forward transmission. Since such a design tends to limit the operating frequency it is known to absorb the reverse wave with a coating of absorbent material of some kind, e.g. graphite, to dissipate the reverse power rather than reflect it.
The spiral, or rather, double spiral, is fed by a balanced line, a twin pair, each of which is connected to a respective spiral termination.
It is known to mount the resulting cavity-backed antenna on a balun as shown in FIG. 1 of the accompanying drawings to convert the balanced twin line of the antenna feed to an unbalanced coaxial terminal port for connection to a transmitter/receiver. While the balun is satisfactory over a limited frequency range it is always desirable to extend the range of operation and/or generally improve the response.